[libunwind] [llvm] [runtimes] remove workaround for old CMake when setting `--unwindlib=none` (PR #93429)

[via llvm-commits]

h-vetinari wrote:

Haha, just when I realized the key to the whole thing - luckily my comment won the race condition at least. ;-)

Thanks a lot for the fix, I've pulled it into the PR (also rebased on main while I'm at it) - and CI didn't fail as previously, so this looks great! Thank you!

https://github.com/llvm/llvm-project/pull/93429